The present invention relates to reduction of flicker of an optical display, and more particularly, to reduction of flicker for a stretched gram of a sonar display.
To avoid objectional flicker in a cathode ray tube display, it is customary to have the horizontal components of the characters at least two lines wide. In this manner, because of the 2:1 interlace which is the standard, the perception of flicker by the eye is avoided.
Sonar displays are generated by presenting gray scale data in an interlaced raster format on an optical display. Each line of the display is divided into picture elements called pixels with each of the pixels being displayed at four (ASP) or eight (BQQ-5) intensity levels. The data is refreshed at 20 Hz (ASP) or 26 Hz (BBQ-5) and interlaced 2:1 to avoid the perception of flicker by the operator. This interlacing results in the display of two fields per frame so that field rate is twice the frame rate. With interlaced raster formats, one field is composed of designated odd raster lines and the other field is composed of designated even raster lines. When adjacent data is displayed on both fields, the eye perceives the refresh to be at the field rate instead of at the frame rate. Since the field rate is above a critical flicker fusion frequency, the display appears to be flicker free.
However, when isolated data appears on only one horizontal line, the interlacing has no effect, and the eye perceives the data as being refreshed at the frame rate and thus detects flicker. For alphanumeric data, this problem has been solved by composing character fonts with data placed on both the odd and even fields which is called double dotting. If the line goes on and off at the 30 cycle rate it could be annoying to most viewers. However, if two lines adjacent to each other are alternately turned on and off, no large area of flicker will exist and the effective flicker rate for the large area becomes twice the previous rate. In this situation, horizontal vectors have been doubled to minimize the perception of distracting flicker.
In the usual case, a CRT display is used for showing a plurality of display points and the flicker of any particular portion of the display is reduced when viewed simultaneously with non-flickering portions of the display. Thus, in an optical display having a complex image, the apparent flicker of any one point is reduced because it is simultaneously viewed with non-apparent flickering portions of the raster thus reducing the effect of the flicker by viewing the flickering portion in proximity with another non-flickering portion. However, this is not the case for the optical display of acoustic data such as exhibited in a sonar display.
In some sonar displays the data presented on the screen approaches that of an isolated line which therefore does not receive the benefit of the visual illusion of the eye for reducing flicker when viewed in proximity with non-flickering portions of the display. Acoustic data which is received for display is often in the form of a single gram which for visual optical display purposes is stretched redundantly in the horizontal direction with each pixel becoming a horizontal line of 2, 4, 8 etc. pixels wide. As the acoustic data is redundantly stretched with such data occupying a larger portion of the horizontal scan of a single field, such a display defeats the effect of the interlace as much as the stretched redundant line is displayed on one line of one of the fields and does not receive the benefit of the interlace effect. Thus, the operator perceives flicker. Resort to double dotting as a redundancy of duplicating the stretched gram on a proximate line of the other raster field would cause the display data to occupy two adjacent raster lines and cause a significant loss of resolution.
Accordingly, it is desirable to provide a means for the optical display of acoustic data wherein the visual perception of flicker is minimized. It is also desirable to provide a means for reducing the visual perception of the flicker on an optical display wherein loss of resolution is minimized.